Cementitious compositions are used to form a wide variety of products, including construction supports, sidewalks, building and decorative tiles, and the like. Generally these compositions comprise a base material, such as Portland cement or calcium aluminate, an inorganic filler, and other additives which influence the physical properties of the composition. Cementitious compositions are attractive to manufacturers and consumers because of their strength, hardness, heat resistance, ease of forming into a desired shape, and relatively low cost.
Recently, interest has developed in using cementitious compositions in prototype molds. Typically, prototype molds are used for the production of a limited number of prototype parts for testing and analysis; as a result, mold-makers generally look for an inexpensive material from which to produce the mold. In addition, prototype mold materials should be hard, easily formed into a potentially complicated and detailed shape, capable of providing a glossy surface, and heat resistant. Commonly used materials are soft metals, such as aluminum, and fiberglass. However, aluminum must be machined into the desired shape, and fiberglass must be hand-laid. In contrast, cementitious materials can be molded easily into the desired shape without the labor-intensity incumbent to aluminum and fiberglass molds, while still satisfying the performance criteria at a low cost.
General purpose cementitious compositions, particularly those based in Portland cement, that have been tried in molds have demonstrated two problems. First, the full curing time of cementitious molds based on Portland cement compositions is typically about 30 days. Prototype molders generally desire a much shorter mold production cycle. In a typical design cycle, a molded item will undergo several design changes before a final design configuration is chosen. Each of these different designs may require the molding and testing of the item in that design. As a result, the turnaround time for mold construction is a major determinant of the duration of the design phase of a project. A material which requires 30 days to cure is thus unsuitable for use in many prototype molds.
Calcium aluminate compositions are generally understood to cure more rapidly than Portland cement compositions, which makes them more viable candidates for use in prototype molds when compared with Portland cement compositions. However, a rapid cure cycle leads to a related concern: a composition with an acceptably brief cure time also typically has an unacceptably short pot life; i.e., it sets up so quickly after mixing that forming it into a desired shape, particularly a complicated shape with considerable detail, is difficult at best. Spensler U.S. Pat. No. 4,455,171 describes a calcium aluminate composition which includes both a setting inhibitor and a reactivator in an attempt to balance the requirements of long pot life and short cure time; however, the combinations disclosed are suitable for extending the shelf life of a composition for several months rather than for a matter of minutes. The prior art is silent on a composition which has a pot life of at least 15 minutes yet cures within 12 to 36 hours.
The second difficulty encountered with cementitious compositions is the tendency of these compositions to develop microcracks in the mold surface which in turn mar the surface of a molded item. This microcracking is exacerbated by exposure of the mold to elevated temperatures. As a result, molds made of Portland cement compositions are not suitable for use in molds which produce parts for which surface appearance is critical. The prior art is silent as to compositions which address or alleviate surface microcracking.
Therefore, there continues to be a need for a cementitious material suitable for use in molds. Such a material would have a pot life which permits forming of a desired shape prior to set-up, a short cure time to permit rapid testing and re-testing of designs, and virtually no surface microcracking.